The invention is more particularly concerned with methods and apparatus for monitoring the condition of assemblies such as a gas-turbine engine in an aircraft.
It is known to monitor the vibration produced by engines and bearings and to detect when certain frequencies of the vibration exceed a predetermined level so as to establish when wear or damage has been produced. This can be a useful technique but is only capable of detecting significant amounts of wear or damage. This is because the vibration spectrum produced by the engine or bearing varies normally according to its operational condition such as speed, temperature, loading and so on. The amplitude above which one frequency of vibration will trigger a wear or damage signal must be sufficiently high that it is not triggered at the maximum amplitude experienced in normal use. This makes the apparatus relatively insensitive to incipient damage. Furthermore, although previous systems relying on the detection of spectral peaks can be used to identify known faults which cause spectral peaks at predictable frequencies, they are not capable of responding to more subtle spectral variations, such as distortion not involving sharp peaks, or unusual combinations of peaks, or unusual sequences. The response of previous systems is therefore severely limited as far as incipient or hitherto unknown fault conditions are concerned.